1. Field of Invention
This invention relates to a high resolution optical scale reader for measuring a distance of movement of a reflective scale by utilizing interference of diffracted beams of coherent light and measuring a direction of movement of the scale by utilizing a phase delay of the beams caused on reflection on a half mirror, and more particularly, to such an optical scale reader which, for example, can be used effectively as a position detector for positional detection in the range of submicrons, in a VLSI manufacturing apparatus, electron beam exposure apparatus, and the like.
2. Description of the Prior Art
Known in the art are a variety of optical scale readers using the principle of interference of light. FIG. 1 illustrates a conventional optical scale reader. A laser beam l emitted by a laser beam source LE is reflected by a mirror M1, and travels through lenses L1,L2 onto a scale 1. A +1st order diffracted beam of light which is diffracted by scale 1 hits a mirror M2, and a 0th order diffracted beam of light hits mirror M3. The +1st order diffracted beam of light reflected by mirror M2 is then radiated onto a lense L3. The 0th order diffracted beam of light reflected by mirror M3 is diffracted by scale 1 and falls as a -1st order diffracted beam of light on lense L3.
At this time, the +1st order and -1st order diffracted beams of light are converted by respective polarizers P1, P2, into linearly polarized beams of light having planes of polarization which are angularly displaced 90.degree. from each other. The beam of light combined by lens L3 is divided by a beam splitter 2 into three beams which are directed to photoelectric transducers D1,D2 and D3, respectively. Photoelectric transducer D3 generates an output signal which is employed for automatic gain control to keep laser beam l constant. The light beams falling on photoelectric transducers D1, D2 are 90.degree. out of phase with each other because of a 1/4 wave plate 13 preceding the photoelectric transducer D2. The +1st order diffracted beams of light are mixed by analyzers P3,P4 preceding the transducers D1,D2 and having planes of polarization which are angularly displaced 45.degree. from each other. The beams of light, with interference fringes, are converted into electric signals by transducers D1,D2. The output signals from transducers D1,D2 are then processed into a value indicative of a distance which scale 1 has moved.
The scale of the foregoing optical reader arrangement is light transmissive and cannot be easily moved. The prior optical reader has been complex in construction, requiring polarizers, analyzers, 1/4 wave plates, and beam splitters for bringing the light beam 90.degree. out of phase.